The present invention relates to a display system for a computer or word processor using a display screen which is divided into a plurality of display areas where different images are respectively displayed.
For scrolling an image (including characters and figures) on a display screen, conventional display system shifts a start address at which data is started to be read out from a display data RAM by a preset amount (corresponding to one line). When the display screen is divided into two areas and the display image in each area is to be scrolled, the following drawback occurs. When the sets of the display data for each divided area of the screen are both stored in one display data RAM and the starting address for reading out data is shifted greatly for scrolling, data of the other area of the screen will be read out. This causes confusion between the areas of the screen when scrolling. To avoid this confusion, respective display data RAMs are provided for respective areas of the screen, which leads to drawbacks of complicated configuration of the circuit board, increased the size of the board and subsequently vulnerability to electrostatic noise and AC line noise.
One method for avoiding the increase in the number of display data RAMs for respective scrolling of plural areas of a display is to completely rewrite the content of one area of a display data RAM shared by the plural areas by using the cycle stealing method. Every time a scroll is desired in the display of one of the areas, the whole content of the area of the display data RAM is completely rewritten. But this method has its own drawback that the working time of the CPU for rewriting the content of the area of the RAM is long and accordingly other processes to be executed by the CPU are delayed. Further, a latching circuit is needed to execute the cycle stealing method and the display data RAM having responsiveness higher than a normal RAM is needed in order to shorten the rewriting time.